1. Field of the Invention
This invention relates generally to earphones, and, more particularly, is directed to improving the acoustic characteristics of open-air type earphones in the low and high frequency ranges.
2. Description of the Prior Art
Open-air type earphones according to the prior art have a housing with a driver unit therein comprising a magnetic circuit and a vibration system constituted by a diaphragm and voice coil. In such open-air type earphones, the response decreases at frequencies below the resonant frequency of the vibration system and, therefore, the resonant frequency needs to have a low value in order to improve the low frequency characteristic. In order to reduce the resonant frequency, it is necessary to increase the compliance and/or the equivalent mass of the vibration system. In order to increase the compliance of the vibration system, it is necessary to select a material for the diaphragm having a high compliance, and to decrease the thickness of the diaphragm. However, there are limits to the compliance of the material that can be used for the diaphragm and the extent to which the thickness of the diaphragm can be reduced is also limited. Further, increasing the equivalent mass of the vibration system causes deterioration of the sensitivity and the acoustic characteristic of the earphone in the high frequency range.
In order to avoid the above problems, the present applicant has earlier proposed, for example, as disclosed in Japanese Utility Model application No. 71055/1983, which was the subject of Japanese Utility Model unexamined publication No. 177287/1984, that an in-the-ear earphone or headphone be provided with a duct extending from the housing in back of the driver unit so that, when the earphone is situated in the ear, the duct projects out of the concha. The duct is formed to provide an equivalent mass added to the equivalent mass and compliance of the vibration system so that the resonant frequency is lowered in correspondence to the added equivalent mass of the duct. Therefore, the resonant frequency can be lowered irrespective of the compliance and equivalent mass of the vibration system with the result that the characteristic of the earphone in the low frequency range can be improved. However, such improvement of the characteristic in the low frequency range requires that there be a significant acoustic resistance in parallel with the equivalent mass of the duct, for example, by providing acoustic resistance material in an opening or openings formed in the housing in back of the driver unit. However, in that case, a resonant circuit is formed by the mass of the vibration system and the compliance of the housing in back of the driver unit which is in parallel with the mentioned acoustic resistance. As a result of such resonance circuit, a peak appears in the frequency characteristic of the earphone at frequencies of 3 to 5 kHz, that is, a part of the high frequency range is emphasized, so that metallic sounds become overly conspicuous and unpleasant to hear. In other words, when the low frequency range of the earphone is extended, an undesirable peak occurs in the high frequency range and, conversely, when it is attempted to suppress such peak, the reproducible low frequency range cannot be extended.